This invention relates to methods for the polymerization of monomers, and to novel microemulsion systems for conducting such polymerizations.
Emulsion polymerization is an important commercial process because, in contrast to the same free-radical polymerization performed in the bulk, molecular weight and reaction rate can be increased simultaneously..sup.1 Furthermore, the lower viscosity of an emulsion system compared to that of the corresponding bulk process provides better control over heat transfer. Commercial emulsion processes usually use a surfactant-water-monomer system which is stabilized by vigorous stirring. The dispersed phase contains micelles, approximately 10 to 50 nm in diameter, as well as monomer droplets. In the absence of agitation, these monomer droplets will coagulate and separate as a second phase. If, as is the usual practice,.sup.1 a continuous-phase soluble initiator is used, polymerization commences at the micelle interface and proceeds within the micelles. During the reaction, monomer diffuses from the large droplets into the micelles. Exhaustion of these monomer reservoirs signals the end of the polymerization.
Recently,.sup.2,20,21 polymerization in microemulsions has been studied. In contrast to the emulsion system described above, a microemulsion is thermodynamically stable, and thus one-phase and optically clear in the absence of agitation. Microemulsion polymerization has been used to produce stable lattices with a very fine (approx. 50 nm) particle size..sup.4
Most emulsion polymerization systems employ an oil-soluble monomer dispersed in an aqueous continuous phase. Recent work.sup.2-8 describes polymerizing water-soluble monomers in an inverse emulsion (a water in oil emulsion). However, conventional inverse emulsions are even less stable than conventional water-in-oil emulsions,.sup.1. Although an inverse microemulsion polymerization is an efficient way to produce high molecular weight polymer, there remains the problem of separation of the polymer from a large volume of oil.